PaymentRequest model fixes

[Josiah-J]

1. The integration field of the model should be optional because it is missing in some responses.
2. The notifications field can also be a str type. This may to be a bug on Paystack side because contrary to their documentation, what they actually return is notification: "[]".

In the meantime I just inherited the model with these modifications and it works:

class AlternativePaymentRequestModel(PaymentRequest):
    integration: int | Integration | None = None
    notifications: list[PaymentRequestNotification] | str

[gray-adeyi]

@Josiah-J Thanks for this will address this in a bit. It's nice you're using an alternate model, so you're not blocked. It's not really easy trying to model the response models correctly because of these discrepancies. I'll apply this your fix in a patch soon